Means for handling altitude information in collision avoidance systems

ABSTRACT

An apparatus for improving the handling of altitude information in collision avoidance systems and the like including means to minimize the possibility that a command maneuver to climb or dive to avoid being on a collision course will cause the involved aircraft to fly through each others altitude, said apparatus also including means to improve the reporting of altitude information in collision avoidance systems so that actual as well as shifted altitude information will be transmitted when a maneuver is commanded to forewarn other aircraft of possible dangerous situations developing as a result of the maneuver.

United States Patent [19] Perkinson 1 1 Feb. 6, 1973 54] MEANS FORHANDLING ALTITUDE 3,310,806 3/1967 Stansbury ..343/112.4 F M TI N INCOLLISION 2,649,540 8/1953 Homrighous..... ....179/15 BZ AVOIDANCESYSTEMS 2,933,726 4/1960 Campbelletal. ,..235/150.23

3,341,812 9/1967 Perkinson et a1 ..340/23 [75] inventor: Robert E.Perkinson, St. Louis County, Primary Examiner-Kathleen H. Claffy [73]Assignee: McDonnell Douglas Corporation, St. AssistantEXami'Ie'TTThOma-S Kunde" Louis, Mo. Attorney-Charles B. l-laverstock[21] Appl' L907 An apparatus for improving the handling of altitudeinformation in collision avoidance systems and the like [52] U.S.Cl...340/27 R, 340/23 including means to minimize the possibility that a[51] Int. Cl. ..G08g 5/04 command maneuver to climb or dive to avoidbeing [58] Field of Search .....340/23, 27; 343/75, 1 12 CA; on acollision course will cause the involved aircraft to I 325/19; 179/15 BZfly through each others altitude, said apparatus also including means toimprove the reporting of altitude [56] References Cited information incollision avoidance systems so that actual as well as shifted altitudeinformation will be UNITED STATES PATENTS transmitted when a maneuver iscommanded to 3,097,354 7/1963 Blownsky ..343/6.5 forewarn other aircraftof possible dangerous situa- 3,412,399 11/1968 Chisholm ..343/6.5 tionsdeveloping as a result of the maneuver.

3,217,321 11/1965 Cox, .lr..... ..343/l12.4

3,262,111 7/1966 Graham ..340/23 11 Claims, 1 Drawing Figure /6 INTRuoERABOVE MINIMUM RANGE OR lllL... .l 1 RANGE/RANGE RATE 'I FF Z2 DOWNWARNING a ALTITUDE THREAT to "EPW 01311 G AY a 1 5 SIGNAL F F INVERTER4% p Q. A J INTRUDER 5 LOW a 5 20 28 UP WARNING CONTROL 5. LIGHT DISPLAYDOWN WARNING 1, DOWN WARNING SIGNAL UP WARNING ALTITUDE RAMP RESET UPWARNING ALTITUDE COUNTER PATENTED FEB 6 I975 mwkzaou month? poJm M0332 230 Na ha The present invention relates generally to improvements incollision avoidance systems and the like and moreparticularly toimprovements in systems such as the system disclosed in Perkinson et al.US. Pat. No. 3,341,812, which issued Sept. 12, 1967, and is assigned toApplicants assignee.

It is a principal object of the present invention to provide improvedand more reliable means for reporting and handling altitude informationin collision avoidance systems.

Another object is to minimize the possibility of commanding aircraftthat represent threats to each other to change altitudes in directionssuch that any two of the threatening aircraft will pass through the samealtitude as any of the other threatened aircraft.

Another object is to provide means for transmitting altitude informationwhich takes into account and projects information as to maneuvers whichare commanded or in process and which affect the altitude of thetransmitting aircraft. 1

Another object is to provide meansfor transmitting shifted altitudeinformation from which aircraft flying at other altitudes can determinewhether they will be on a collision course with the transmittingaircraft.

Another main object is to provide means to make flying safer.

Another object is to make sure that all aircraft flying at or near thesame altitude and representing possible collision threats to one anotherare given appropriate escape maneuver commands which will prevent thepossibility of collision.

Another object is to provide relatively inexpensive means, for use inconjunction with known collision avoidance systems to increase theprotection provided by the system.

Another object is to incorporate in collision avoidance systems for useon aircraft improved means for protecting the aircraft during times whenone or more aircraft is changing altitude.

Another object is to minimize the amount of maneuvering necessary foraircraft to escape from potentially dangerous situations.

Another object is to reduce or eliminate the possibility that two ormore aircraft flying at or near the same altitude will be given the sameor similar instructions to avoid a collision.

Another object is to prevent the giving of ambiguous warnings andinstructions in aircraft equipped with collision avoidance equipmentparticularly with regard to altitude information and regardless of thenumber of aircraft involved.

These and other objects and advantages of the present invention willbecome apparent after considering the following detailed specificationwhich covers a preferred embodiment thereof in conjunction with theaccompanying drawing, wherein:

The FIGURE is a circuit diagram in block form of a warning controlcircuit for a collision avoidance system constructed according to thepresent invention.

One of the main problems in most known collision avoidance systems isthe problem of transmitting altitude information between cooperatingaircraft in a manner to enable the various aircraft to determine notonly when two or more aircraft present threats to one another but alsoto produce appropriate command maneuvers to avoid collision and at thesame time to protect against the possibility that a climbing or divingaircraft will present a threat to an aircraft at another altitude. Analtitude threat exists when two or more air craft are, or soon will be,flying at or near the same altitude on courses that are bringing themcloser together. The means and method for determining when two or moreaircraft are on a collision course is discussed in detail in ApplicantsUS. Pat. No. 3,341,812. When it is determined that aircraft are a threatto one another, the involved aircraft should be given appropriate timelyinstructions or maneuver commands so that they can avoid or escape fromthe threatening conditions. Various ways have been proposed and variousmeans devised to accomplish this including giving complementary up anddown or right and left turn maneuver instructions. One way ofimplementing this is applicable particularly to a synchronized collisionavoidance system wherein each cooperating aircraft includes timesynchronized means as disclosed in Perkinson et a1. U. S. Pat. No.3,341,812. In such a system each. cooperating aircraft is assigned adistinct message slot which occurs at the same exact time in eachrepeating time interval and during each occurrence of its message slotit transmits information that identifies it and indicates its altitude.The same information can also be used to determine range and range rate,all of which information is evaluated in each aircraft to determine if athreatening condition exists. In the known systems when a collisioncourse is indicated an arbitrary assignmentof complementary commandmaneuvers is made usually based on some criteria such as on which of theaircraft has the lowest (first to occur) and which the highest (latestto occur) assigned message slot. For example, an aircraft assigned alower numbered message slot may arbitrarily be directed to dive while anaircraft having a higher numbered message slot may 1 at the same time bedirected to climb. Since each aircraft is assigned a distinct messageslot such an arbitrary arrangement will assure that when two aircraftare threats to each other they will be given complementary instructions.However, an arbitrary arrangement :such as described has certaindisadvantages and limitations which make it inadequate and unacceptablein certain situations. In particular, such a system when applied to acollision avoidance system that requires establishing arbitrary limitsas to what will be and what will not be considered a coaltitudecondition has certain disadvantages. For example, for the limits of acoaltitude condition to be reasonable they should cover an altitude bandthat may be as wide as six or seven hundred feet or more in a typicalsituation. Under these conditions and using the time of occurrence ofassigned message slots as the test, an aircraft flying six or sevenhundred feet lower than another coaltitude aircraft but havinga highermessage slot would be instructed to climb while the higher flyingaircraft would be instructed to dive. This is obviously undesirable andis one situation that the present means are designed to overcome. Thearbitrary complementary command system also has disadvantages andlimitations in situations where more than two aircraft representcoaltitude threats to each other. With the possible exception ofsituations where one aircraft is descending or climbing more or lessdirectly above or below another aircraft, the principal disadvantagethat can occur from a system that gives complementary commands based onsome arbitrary criteria when only two aircraft are involved is excessivemaneuvering. However, if a system that gives arbitrary complementarycommands is applied to a situation where two aircraft are flyingdirectly above and below one another within a warning band representinga coaltitude condition for purposes of establishing a collision warning,a dangerous situation may exist even with respect to the two aircraft.

The use of altitude difference instead of time of occurance of messageslots as the basis for the generation of maneuvering commands incollision avoidance equipment has for some time been attractive from thestandpoint of reducing excessive maneuvering and preventing sneakapproaches from above or below. However, the known altitude differencelogic up until the present device has in it the serious disadvantagesoutlined above including particularly the possibility of producingidentical escape maneuver commands in two aircraft flying atapproximately the same altitude. The present means overcome this andother disadvantages and shortcomings of existing equipment, and makeexisting collision avoidance equipment even safer to use.

The present means also have the capability of transmitting altitudeinformation which is projected to reflect a changing altitude situationunder warning or maneuver command conditions as well as during amaneuver involving a change in altitude.

The present means includes means for producing maneuver commands basedon altitude difference, and the commands produced not only assure thatboth aircraft will receive complementary instructions but also that thehigher aircraft will be commanded to climb and the lower to dive therebyminimizing the amount of maneuvering and providing an added safetyfactor so that there will be practically no possibility that theaircraft will pass through each others altitude when making their escapemaneuvers. Another unique feature of the present means is in theprovision of means for transmitting shifted altitude information whichreflects a maneuver decision or command once such a command is given butbefore the command is executed. This is accomplished by transmittingshifted altitude information to warn other aircraft which may not atthat instant be a threat but which will become a threat as a result ofmaking the commanded maneuver which involves a change in altitude. Byshifting the altitude transmissions to reflect the position of thecommanded aircraft at some future time, other aircraft which may bethreatened as a result will have an opportunity to be forewarned evenbefore the aircraft making the altitude change reaches its new altitude.In situations where two or more aircraft are potential threats to eachother because they are flying in the same altitude band but near theextremes thereof, a shifted altitude signal generated by one or more ofthe aircraft may also actually make the generation of a maneuver commandunnecessary thereby reducing the number of required evasive maneuvers.Under the same circumstances, the aircraft that transmits the shiftedaltitude information will continue to determine the need for itsmaneuver command based on its actual altitude separation from otheraircraft, and only when a safe altitude separation has been achieved,will the means for producing the maneuver commands be turned off so thatthe transmissions of unshifted altitude information can be resumed.

ln situations where three aircraft are potential coal-v titude threatsto each other, the aircraft flying at the center or inbetween altituderelative-to the others will receive both up and down warnings on itscontrol panel, and under these conditions no evasive maneuver will becalled for by it, and hence no shifted altitude transmissions will takeplace. Where more than three aircraft are involved in a potentialaltitude conflict, the same general principles apply, and can be handledby the present means as will be described hereinafter.

The FIGURE shows a circuit which includes two similar flip-flop circuitsl0 and 12, each of which has input and output connections. The flip-flopcircuits must at all times be in one of two operating conditions, thatis to say when one of the circuits is in one of its two operatingconditions the other circuit must be in its opposite condition. By thesame token when a signal is received which causes a change in thecondition of one of the flip-flop circuits, the other flip-flop willsimultaneously change but in an opposite sense. It should also beunderstood that a similar circuit is included in each cooperatingaircraft, and in the discussion which follows the word own will be usedto refer to one aircraft andthe words intruder or intruders to otheraircraft.

The flip-flop 10 in each aircraft has a first input 14 which isconnected to receive input signals which represent the electricalinverse or mirror image of its own altitude which is the altitude of theaircraft in which the system is located. The inverse form of the signalis represented by the line drawn over the expression Own Altitude", andmeans that an input will be present at the input 14 at all times exceptwhen the input is identical with the aircraft's own altitude. OwnAltitude" is represented by a pulse which occurs after the beginning ofa received message by a time interval that is proportional to thealtitude of the own aircraft, and the time of occurrence of the altitudepulse is determined by means carried in the aircraft. Such means mayinclude pressure sensitive means, ground bounce means, or other likemeans all of which are well known devices for determining and indicatingaltitude.

The circuit in the FIGURE receives input altitude signals which are theresult of measurements made by the altitude measuring means in its ownaircraft as well as altitude signals similarly produced and transmittedfrom other aircraft in their assigned message slots. The present systemoperates as part of a time synchronized system in which each cooperatingaircraft is assigned a distinct message slot which reoccurs at the sameexact time in each repeating period. For example, a time period may lastfor several seconds and be divided into message slots of a fewmillisecond durations. For example, if each message slot is of twomilliseconds duration there will be 500 message slots in each second ofeach time period. Within its assigned message slot each aircrafttransmits, among other things, an altitude pulse which commences at somepredetermined time after the end of a range pulse which may be of 200microseconds duration and commences at the beginning of the messageslot. The time 1 duration between the end of the 200 microsecond rangepulse and the beginning of the altitude pulse will then be proportionalto the altitude of the aircraft. All cooperating aircraft will doexactly the same thing except that each aircraft will transmit thisinformation only in its assigned message slot but will use this sameinformation in the aircraft to make altitude comparisons in everymessage slot in which it receives a transmission from another aircraft.Each aircraft will therefore transmit information as to its own altitudeonly in its assigned message slot, and will receive similartransmissions from other aircraft in their assigned message slots. Adirect comparison of the altitudes of the transmitting and receivingaircraft can then be made using the subject circuits includingparticularly the flip-flops and 12 as stated. However, before anymeaningful outputs will be produced by the present means it is firstnecessary for all of the criteria to be satisfied as to the existence ofa warning condition. As already mentioned some of the inputs to theflip-flops 10 and 12 are inverted inputs or electrical mirror images ofthe actual input signals, but this does not change the basic operatingcharacteristics and is done more for convenience than for any otherreason.

The flip-flop 12 has another input 16 which is connected to receiveinput signals representing the inverse of thealtitude of an intrudingand potentially threatening aircraft. The flip-flops 10 and 12 also haveother input connections 18 and 20 respectively, which are connectedtogether and to a source of timed input signals identified in thedrawing as timing pulses t These pulses are used to reset the flip-flopsl0 and 12 to some predetermined reset condition at the beginning of, orimmediately prior to the start, of each message slot. When a t impulseresets the flip-flops 10 and 12 at the beginning of each message slot, aramp generator or like means are started which continue counting untilstopped by receipt of a pulse at the terminal 14 or 16 which everhappens to arrive first. As already stated, the altitude of eachaircraft is represented by the time interval between the start of itsassigned message slot (or the end of its range pulse) and the time ofoccurrence of the leading edge of the altitude pulse which occurs in itsmessage slot. There can be any desired number of message slots in therepeating time periods depending upon the capacity of the collisionavoidance system, and the flip-flops l0 and 12 will therefore be resetmany times during each operating cycle or time period. For example,there may be several thousand or more message slots in each time period,and each cooperating aircraft will be assigned a distinct one of themessage slots which will occur at exactly the same time in 'each of thetime periods.

The first altitude pulse to arrive at the flip-flop 10 or 12 in aparticular message slot will trigger the associated flip-flop and in sodoing will provide a logic output signal to indicate whether theintruding aircraft is flying at a higher or lower altitude than the ownaircraft. For example, if each cooperating aircraft is assigned amessage slot of say two milliseconds duration, the altitude pulse whichoccurs nearest to the beginning of the range pulse will represent theaircraft that is flying lower. This requires that during every messageslot in which a message is received, the own aircraft produces its ownaltitude pulse, timed with respect to the beginning of the receivedrange pulse, for comparison with the received altitude pulse of thecooperating aircraft. This means that if both flip-flops 10 and 12 arereset at the same time beginning of each message slot, then the firstaltitude pulse received or generated, be, it the own aircrafts altitudepulse or the altitude pulse received from another or intruder aircraft,will indicate which of the two aircraft is flying at the lower altitude.It is also possible to make the altitude pulse which occurs nearest thebeginning of the range pulse represent the higher flying craft withoutdeparting from the invention. Furthermore, because of the cross-couplingconnections 22 and 24 between the flips-flops l0 and 12 the firstflip-flop to be triggered will operate to lock the other untriggeredflip-flop in its untriggered state. The above/below informationcontained by the two flip-flop circuits 10 and 12 can then be used inthe warning logic for determining what, if any, excape maneuver iscalled for and/or commanded. For example, if the flip-flop 10 receivesits own altitude signal before the flip-flop 12 receives the intrudersaltitude signal, the flip-flop 10 will be triggered, and will produce anoutput at its output terminal 26 indicating that the intruder is flyingat a higher altitude than the own" aircraft. By the same token, if theintruder aircraft altitude signal occurs first thereby triggering theflip-flop 12 and locking the flip-flop 10 in its untriggered condition,an output signal will be produced on the output terminal 28 of theflip-flop 12 indicating that the intruder if flying at a lower altitudethan the own aircraft. It is important to recognize thatbefore thepresent circuit can operate and produce meaningful outputs it must firstbe determined that the aircraft involved are threats to each other andthis means, among other things, that they are flying at approximatelythe same altitude within some preestablished altitude band which is anarbitrary coaltitude band for collision avoidance purposes. The meansfor making this determination are described in Perkinson et al. US. Pat.No. 3,341,812. It is also important to recognize that regardless of howclose to the exact same altitude the aircraft are flying one will alwaysbe indicated by the present means as being the higher and the other thelower. This is so because it is only possible for one of the flip-flops10 and 12 to be triggered first in any assigned message slot. Theoutputs of the flip-flop circuits are used to energize appropriateindicator means, providing there is a threat as determined byRange/Range Rate and Altitude sorting signal 74, said indicator meansshown as the up warning and down warning control and light displays 64and 70, respectively, the display portions of which are convenientlylocated in the cockpit. The warning control and displays 64 and areusually energized for some predetermined time period such as for aperiod equal to the time period required for each repeating cycle ofoperation of all of the message slots.

The subject circuit also includes a gate maze circuit which is shown inthe FIGURE constructed of three NAND gates 30, 32 and 34. The NAND gatesare connected to respond to the warning control and light displays 64and 70, andthe NAND gates produce other outputs which can be used forpresetting an altitude counter or ramp generator 35 which may be part ofthe collision avoidance system of which the present device is a part andused for normal or shifted altitude reporting and/or transmitting usingthe transmitting means and associated antenna means 37 which are showngenerally.

The NAND gate 30 has four separate input connections 36, 38, 40 and 42and an output connection 44. The input connection 36 is connected toreceive up warning inputs from the output of the Up Warning Control AndDisplay 64, the input connection 38 to receive inverted or not downwarning signals from the output of the Signal Inverter 66, the inputconnection 40 receives the aircrafts own message slot information, andthe input connection 42 receives reset inputs. It is the nature of NANDgates that they produce output signals under all conditions except whenthere are inputs simultaneously present at all of their inputconnections. This means there will be outputs from the NAND gates 30 atall times when there are input signals at less than all of the inputs aswell as when there are no signals at any of the inputs. By the sametoken there will be no signal at the output connection 44 of the NANDgate 30 when there are inputs simultaneously present at all four of theinput connections 36-42. The electrical characteristics and features ofthe present means involve matters of good engineering practice, and theparticular embodiment shown using NAND gates is a convenient one butthis selection is not at the heart of'the invention.

The NAND gate 32 is similar to the NAND gate 30 in that it has fourinput connections 46, 48, 50 and 52 and one output connection 54. In thecase of the NAND gate 32, however, the input connection 46 receives downwarning output signals from the down warning control and light display70 instead of from the up warning control and light display 64 andinverse or not up warnings from the output of the Signal Inverter 72.

The NAND gate 30 produces output 56 which operates to reset the altitudegenerator or counter 35 at the own aircraft during a climb warningperiod when the climb arrow in the up warning and light control display64 is energized. These signals can also be used to delay the start ofthe altitude ramp generator for transmission of higher than actualaltitude output signals which will enable other aircraft in the areaflying at higher altitudes to determine in advance and in the usual waythe future existence of a possible dangerous condition which will occuras the own aircraft executes its climb maneuver. In similar manner, theNAND gate 32 senses the condition of a down maneuver being flashed tothe pilot and generates outputs on another lead 57. These outputs can beused to advance the starting time of the altitude ramp generator fortransmission of lower than actual altitude condition signals which takesinto account the indicated and commanded dive maneuver. These operationsare important for the present device to enable it to advise otheraircraft as to what its altitude will be at some future time as a resultof a climbing or diving maneuver that is or is about to be made and toprevent other aircraft from making the same decision.

The NAND gate 34 has three input terminals 58, 60 and 62 with theterminal 58 connected to receive the outputs of the NAND gate 30, theterminal 62 connected to receive the outputs of the NAND gate 32, andthe terminal 60 connected to receive the same reset impulses which arealso fed to the terminals 42 and 52, respectively, of the NAND gates 30and 32. The NAND gate 34 produces outputs during altitude counter resetsanytime that neither of the NAND gates 30 and 32 is generating anoutput. The NAND gates 30 and 32 cannot simultaneously generate outputsbecause of the automatic lockout feature provided by the Up and Not Downinputs to the gate 30 on input connections 36 and 38, respectively, andthe Down and Not Up inputs to the gate 32 on the input connections 46and 48. This means that in an aircrafts own message slot, the NAND gate34 will produce an output pulse for normal counter resetting if nowarning is being flashed to the pilot, or if both up and down warningsare simultaneously being energized and displayed but not underconditions when there is only an up or down command. In all messageslots other than own message slot the NAND gate 34 will produce anoutput for normal resetting of the altitude counter regardless ofwhether the own pilot is receiving a command. Furthermore, once thecriteria required for generating a command maneuver no longer exist, thepresent circuit will reset to a standby condition in its own slot andthe NAND gate 34 will produce normal reset output pulses.

The outputs of the three NAND gates 30, 32 and 34 can be used in anydesired combination for predeterminately resetting of a digital altitudecounter or for adding to or subtracting from an arbitrary voltage suchas is used in a ramp generator, in either case to shift the altitudesignal in a desired direction. Resetting of the altitude generator cantake several different forms depending on the form of generator used.For example, if a digital altitude counter is used it can be reset tosome predetermined initial number of counts and in the case of an analogaltitude generator by a shift in the initial voltage level.

In the FIGURE, an intruder above output on the lead 26, as alreadystated, is also used to energize down warning and light display in theown cockpit to alert the pilot to the fact that there is an intrudingaircraft flying at a higher altitude and representing a present danger.This is the same signal that is fed to the down warning input connection46 of the NAND gate 32 and to inverter circuit 66 by way of the downwarning control and display 70 which has its output connected by lead 68to the inverted down or not down warning input connection 38 of the NANDgate 30. In a similar manner, the output connection 28 of the flip-flop12 is connected to energize the up warning control and light display 64in the cockpit when there is a threat and also to provide signals forthe up warning input connection 36 of the NAND gate 30. The output ofthe up warning control and light display 64 is also connected to anotherinverter circuit 72 which has its output connected to the inverted up ornot up warning input connection 48 of the NAND gate 32. Thus it can beseen that the conditions of the flip-flops 10 and 12, together with athreat situation or signal 74, not only control the energizing of thecockpit warnings for the pilot but also control the inputs to the ANDgates 30 and 32. The conditions of the NAND gates 30 and 32 in turncontrol the type of resets that will occur, and whether the own messageslot reset will receive normal or shifted reset signals during a warningcondition.

It can therefore be seen that the present improvements are designed andconstructed to provide and assure a safe vertical separation betweenaircraft and particularly between aircraft that are determined to bethreats to each other. This is true even in cases where only one of twoor more warned aircraft can or does obey a command to climb or dive. Itis anticipated that in the vast majority of cases involving two fullyequipped aircraft that both aircraft will cooperate by making maneuversas instructed by the commands they receive. However, exceptionalsituations may occur and the present system has built in safety featuresthat assure a safe vertical separation in situations even where only oneof the warned aircraft makes its commanded maneuver. These provisionsalso help to make the system compatible and safe to use as between fullyand partially equipped aircraft. For example, a commercial aircraft maybe fully equipped with a collision avoidance system such as the systemdisclosed in U.S. Pat. No. 3,341 ,812 while a smaller aircraft forexpense reasons or otherwise may have somewhat less than a completesystem and still be able to cooperate with more fully equipped aircraftwith a high degree of safety even though on a somewhat limited basis.

There are therefore several different features of the present systemwhich make it a particular valuable addition to a collision avoidancesystem such as the system disclosed in U.S. Pat. No. 3,341,812. One ofthese features is the ability of the present system to make a veryaccurate determination as to which of two or more aircraft, even thoughflying at or near the same altitude or within the limits of what isconsidered to be a coaltitude condition, is higher or highest and whichis lower or lowest. This is determined by which of the flipflops 10 or12 is triggered. By knowing this it is possible to give maneuvercommands which substantially reduce the possibility that any warnedaircraft will fly through the others altitude to avoid collision. Thisnot only enchances the safety but also reduces to a minimum the amountof necessary maneuvering. The features of the present system whichenable transmission of altitude signals which are shifted to give effectto a commanded maneuver to provide a basis by which to determine iffuture collision conditions may occur as a result of the maneuver, isalso important to the system. Shifted altitude signals can also betransmitted any time an aircraft is climbing or diving, but unless theaircraft is commanded to do so as a result of the existence of a warningcondition from its collision avoidance system it is outside of thenormal scope of this invention and is part of the collision avoidancesystem itself.

In a typical situation where the present means are to be used, avertical separation between cooperating aircraft of more than 800 feetis considered a safe separation and anything less is considered to be acoaltitude condition for purposes of determining whether or not awarning should be given. In this case, "the-800 feet separation issomewhat greater than the usual separation considered to be a safeminimum separation. The

800 feet separation results from the incremental digitizing of thealtitude plus a likely overshoot due to the pilots inability to reducethe altitude rate of change to zero promptly after a vertical avoidancemaneuver command is terminated. The somewhat greater minimum safeseparation required by the present system therefore, provides a measureof added protection, and in addition compensates for pilot overshoot.

There has thus been disclosed novel improvements to collision avoidancesystems and particularly synchronized systems which make the knownsystems safer and more reliable to use: particularly in the evaluatingand transmitting of altitude information. It is apparent, however, thatthe particular embodiment chosen to illustrate the invention is not allinclusive and many changes and variations could be made withoutdeparting from the spirit and scope thereof. For example, many differentforms of flip-flops and gate circuits could be used to mention severalobvious possibilities. All such changes and variations which do notdepart from the spirit and scope of the invention are deemed covered bythe present invention which is limited only by the claims which follow.

What is claimed is:

1. In a cooperative collision avoidance system which operates on realtime and on one-way transmissions between cooperating aircraft andwherein each cooperating aircraft includes time keeping means, and meansfor time synchronizing the time keeping means thereat with similar timekeeping means in other cooperating aircraft, and wherein eachcooperating aircraft is assigned a distinct message slot which reoccursat the same time in each repeating time interval in which to transmitinformation signals, means in each aircraft for receiving saidinformation signals transmitted by other cooperating aircraft in rangethereof from which information signals each receiving aircraft candetermine by receipt thereof whether threat of collision exists betweenit and the transmitting aircraft, said transmitted information signalsincluding information time coded to represent the altitude of thetransmitting aircraft, said means in each aircraft for making adetennination as to whether conditions exists which represent apotential collision threat with another cooperating aircraft includingmeans for making the determination based only on a determination ofrange, range rate and whether the involved aircraft are within apredetermined established coaltitude band with respect to each other,the improvement comprising means at said one aircraft for generating animpulse which represents its present altitude in each assignable messageslot and means for determining by the time of receipt of the first oftwo altitude impulses to occur including its own altitude impulse and animpulse from another aircraft that has been determined to be a threatthereto in the message slot assigned to said other aircraft which of theaircraft is higher and which is lower,

said means including bi-stable means in each aircraft Y operable toascertain by the first of said two impulses to occur in the message slotassigned. to said other aircraft which of the threatened'aircraft isflying at a higher and which is flying at a lower altitude in theestablished coaltitude band, means for shifting the altitudetransmission from each such aircraft in a direction to reflect analtitude that is in the direction in which the said aircraft iscommanded to maneuver so that reciprocal escape maneuver commands willbe produced in the involved threatened aircraft during subsequenttransmissions in the message slots assigned to the aircraft from whichthe shifted altitude signals are transmitted, an up warning indicatorand a down warning indicator located in the cockpit of said oneaircraft, means for energizing said up warning indicator whenever it isdetermined that the said one aircraft is flying at a higher altitudethan the other aircraft after it is also determined that said otheraircraft represents a threat, said down warning indicator beingenergized whenever it is determined that the said one aircraft is flyingat a lower altitude than the other aircraft involved after it isdetermined that there is a threatening condition.

2. In the cooperative collision avoidance system of claim 1 the furtherimprovement of means responsive to the energizing of the up warningindicator to shift the time of transmission of the time coded altitudetransmissions therefrom in a direction to represent a higher than actualaltitude condition.

3. In the cooperative collision avoidance system of claim 1 the furtherimprovement of means responsive to energizing of the down warningindicator to shift the time of transmission of the time coded altitudetransmissions therefrom in a direction to represent a lower than actualaltitude condition.

4. In a cooperative collision avoidance system which operates on realtime, one-way transmissions and wherein all cooperating aircraft areequipped with means for transmitting and receiving signals includingmeans whereby each cooperating system is assigned distinctive repeatingtimes for transmitting signals therefrom, each cooperating aircraftincluding accurate time keeping means and means for maintaining saidtime keeping means in time synchronism with the time keeping means atother cooperating aircraft, and means at each aircraft for determiningfrom each of the signals it receives from another aircraft whether ornot it is on a collision course with the said other aircraft based onestablished criteria as to range, range rate and being in a coaltitudecondition with each other which is represented by the aircraft havingless than a predetermined altitude separation, said transmitting meansin each aircraft including means for transmitting information encoded torepresent the altitude of the transmitting aircraft, the improvementcomprising means for determining at each receiving aircraft whether thealtitude is higher or lower than the altitude of each aircraft whosetransmission it receives in their respective transmitting timesincluding particularly those transmissions from aircraft that have beendetermined to represent collision threats to the receiving aircraft,said last named means including means at each receiving aircraft toproduce a signal during each occurrence of each assignable transmittingtime to represent its own present altitude and bi-stable means capableof being in one of two alternate conditions depending upon the first tooccur in a transmitting time assigned to a threatening aircraft of analtitude transmission from said threatening aircraft or of the signalrepresenting the present altitude of the receiving aircraft, saidbi-stable means being operable to determine which of the threatenedaircraft is flying at the higher and which is flying at the loweraltitude, said bi-stable means producing a first output when thereceiving aircraft in which it is located is the higher of thethreatened aircraft and a second output when the receiving aircraft inwhich it is located is the lower of the threatened aircraft, a firstcommand indicator in each receiving aircraft energizable by occurrencesof the said first outputs threat to command the pilot at the saidaircraft to climb, and a second indicator in each aircraft energizableby occurrences of said second outputs thereat to command the pilot todescend, means for modifying subsequent transmissions from a commandedaircraft to reflect an altitude shifted in the direction of thecommanded maneuver and means in the said other threatened aircraftresponsive to receipt of the modified transmission to produce areciprocal maneuver command.

5. In the cooperative collision avoidance system of claim 4 the firstindicator in the higher flying aircraft will be energized to command thepilot thereat to climb while the second indicator in the lower flyingaircraft will substantially simultaneously be energized to command thepilot thereat to descend in altitude.

6. In the cooperative collision avoidance system of claim 4 whereinthree aircraft are flying on courses that are determined to representcollision threats to each other, the further improvement of means fordetermining in one of said aircraft that it is flying at the highestaltitude of the three aircraft, means for determining in a second one ofsaid aircraft that it is flying at the lowest altitude, said means fordetermining that said one aircraft is flying at the highest altitudeproducing an output to energize the first command indicator thereat, themeans for determining that said second aircraft is flying at the lowestaltitude producing an output to energize the second command indicatorthereat, the third aircraft including means for producing a secondoutput thereat with respect to the highest flying aircraft and a firstoutput thereat with respect to the lowest flying aircraft, said firstand second outputs produced at said third aircraft simultaneouslyenergizing the first and second command indicators thereat.

7. In the cooperative collision avoidance system of claim 4 wherein thebi-stable means in each aircraft for determining which aircraft isflying higher and which is flying lower include a flip-flop circuit.

8. In the cooperative collision avoidance system of claim 4 includingmeans responsive to the energizing of the first command indicator onlyto modify the transmitting means in the associated aircraft so that theaircraft will transmit altitude signal encoded to represent an altitudethat is higher by some predetermined altitude than the actual altitudeof the said aircraft.

9. In the cooperative collision avoidance system of claim 4 includingmeans responsive to the energizing of the second command indicator onlyto modify the transmitting means in the associated aircraft so that theaircraft will transmit altitude signals encoded to represent an altitudethat is lower by some predetermined altitude than the actual altitude ofthe said aircraft.

10. In the cooperative collision avoidance system of claim 6 whereinsaid means in said one aircraft for energizing the first commandindicator thereat include means to modify the transmitting means at saidone aircraft so that the transmitting means will transmit altitudesignals encoded to represent a higher than actual altitude conditiontherefrom, and wherein said means in said second aircraft for energizingthe second command indicator thereat include means to modify thetransmitting means at said second aircraft so that the transmittingmeans will transmit altitude signals encoded to represent a lower thanactual altitude condition therefrom, the simultaneous energizing of thefirst and second command indicators in said third aircraft having noeffect on the encoded altitude signals being transmitted therefrom.

11. In the cooperative collision avoidance system of claim wherein saidmeans to modify the transmitting means to transmit signals representinghigher than actual, lower than actual, and actual altitude in eachaircraft include a flip-flop circuit for use in determiningflip-flopcircuit and outputs which operate to control 7 the transmitting meansthereat to cause transmissions of altitude signals encoded to representhigher than actual, lower than actual or actual altitudes.

1. In a cooperative collision avoidance system which operates on realtime and on one-way transmissions between cooperating aircraft andwherein each cooperating aircraft includes time keeping means, and meansfor time synchronizing the time keeping means thereat with similar timekeeping means in other cooperating aircraft, and wherein eachcooperating aircraft is assigned a distinct message slot which reoccursat the same time in each repeating time interval in which to transmitinformation signals, means in each aircraft for receiving saidinformation signals transmitted by other cooperating aircraft in rangethereof from which information signals each receiving aircraft candetermine by receipt thereof whether threat of collision exists betweenit and the transmitting aircraft, said transmitted information signalsincluding information time coded to represent the altitude of thetransmitting aircraft, said means in each aircraft for making adetermination as to whether conditions exists which represent apotential collision threat with another cooperating aircraft includingmeans for making the determination based only on a determination ofrange, range rate and whether the involved aircraft are within apredetermined established coaltitude band with respect to each other,the improvement comprising means at said one aircraft for generating animpulse which represents its present altitude in each assignable messageslot and means for determining by the time of receipt of the first oftwo altitude impulses to occur including its own altitude impulse and animpulse from another aircraft that has been determined to be a threatthereto in the message slot assigned to said other aircraft which of theaircraft is higher and which is lower, said means including bi-stablemeans in each aircraft operable to ascertain by the first of said twoimpulses to occur in the message slot assigned to said other aircraftwhich of the threatened aircraft is flying at a higher and which isflying at a lower altitude in the established coaltitude band, means forshifting the altitude transmission from each such aircraft in adirection to reflect an altitude that is in the direction in which thesaid aircraft is commanded to maneuver so that reciprocal escapemaneuver commands will be produced in the involved threatened aircraftduring subsequent transmissions in the message slots assigned to theaircraft from which the shifted altitude signals are transmitted, an upwarning indicator and a down warning indicator located in the cockpit ofsaid one aircraft, means for energizing said up warning indicatorwhenever it is deterMined that the said one aircraft is flying at ahigher altitude than the other aircraft after it is also determined thatsaid other aircraft represents a threat, said down warning indicatorbeing energized whenever it is determined that the said one aircraft isflying at a lower altitude than the other aircraft involved after it isdetermined that there is a threatening condition.
 1. In a cooperativecollision avoidance system which operates on real time and on one-waytransmissions between cooperating aircraft and wherein each cooperatingaircraft includes time keeping means, and means for time synchronizingthe time keeping means thereat with similar time keeping means in othercooperating aircraft, and wherein each cooperating aircraft is assigneda distinct message slot which reoccurs at the same time in eachrepeating time interval in which to transmit information signals, meansin each aircraft for receiving said information signals transmitted byother cooperating aircraft in range thereof from which informationsignals each receiving aircraft can determine by receipt thereof whetherthreat of collision exists between it and the transmitting aircraft,said transmitted information signals including information time coded torepresent the altitude of the transmitting aircraft, said means in eachaircraft for making a determination as to whether conditions existswhich represent a potential collision threat with another cooperatingaircraft including means for making the determination based only on adetermination of range, range rate and whether the involved aircraft arewithin a predetermined established coaltitude band with respect to eachother, the improvement comprising means at said one aircraft forgenerating an impulse which represents its present altitude in eachassignable message slot and means for determining by the time of receiptof the first of two altitude impulses to occur including its ownaltitude impulse and an impulse from another aircraft that has beendetermined to be a threat thereto in the message slot assigned to saidother aircraft which of the aircraft is higher and which is lower, saidmeans including bi-stable means in each aircraft operable to ascertainby the first of said two impulses to occur in the message slot assignedto said other aircraft which of the threatened aircraft is flying at ahigher and which is flying at a lower altitude in the establishedcoaltitude band, means for shifting the altitude transmission from eachsuch aircraft in a direction to reflect an altitude that is in thedirection in which the said aircraft is commanded to maneuver so thatreciprocal escape maneuver commands will be produced in the involvedthreatened aircraft during subsequent transmissions in the message slotsassigned to the aircraft from which the shifted altitude signals aretransmitted, an up warning indicator and a down warning indicatorlocated in the cockpit of said one aircraft, means for energizing saidup warning indicator whenever it is deterMined that the said oneaircraft is flying at a higher altitude than the other aircraft after itis also determined that said other aircraft represents a threat, saiddown warning indicator being energized whenever it is determined thatthe said one aircraft is flying at a lower altitude than the otheraircraft involved after it is determined that there is a threateningcondition.
 2. In the cooperative collision avoidance system of claim 1the further improvement of means responsive to the energizing of the upwarning indicator to shift the time of transmission of the time codedaltitude transmissions therefrom in a direction to represent a higherthan actual altitude condition.
 3. In the cooperative collisionavoidance system of claim 1 the further improvement of means responsiveto energizing of the down warning indicator to shift the time oftransmission of the time coded altitude transmissions therefrom in adirection to represent a lower than actual altitude condition.
 4. In acooperative collision avoidance system which operates on real time,one-way transmissions and wherein all cooperating aircraft are equippedwith means for transmitting and receiving signals including meanswhereby each cooperating system is assigned distinctive repeating timesfor transmitting signals therefrom, each cooperating aircraft includingaccurate time keeping means and means for maintaining said time keepingmeans in time synchronism with the time keeping means at othercooperating aircraft, and means at each aircraft for determining fromeach of the signals it receives from another aircraft whether or not itis on a collision course with the said other aircraft based onestablished criteria as to range, range rate and being in a coaltitudecondition with each other which is represented by the aircraft havingless than a predetermined altitude separation, said transmitting meansin each aircraft including means for transmitting information encoded torepresent the altitude of the transmitting aircraft, the improvementcomprising means for determining at each receiving aircraft whether thealtitude is higher or lower than the altitude of each aircraft whosetransmission it receives in their respective transmitting timesincluding particularly those transmissions from aircraft that have beendetermined to represent collision threats to the receiving aircraft,said last named means including means at each receiving aircraft toproduce a signal during each occurrence of each assignable transmittingtime to represent its own present altitude and bi-stable means capableof being in one of two alternate conditions depending upon the first tooccur in a transmitting time assigned to a threatening aircraft of analtitude transmission from said threatening aircraft or of the signalrepresenting the present altitude of the receiving aircraft, saidbi-stable means being operable to determine which of the threatenedaircraft is flying at the higher and which is flying at the loweraltitude, said bi-stable means producing a first output when thereceiving aircraft in which it is located is the higher of thethreatened aircraft and a second output when the receiving aircraft inwhich it is located is the lower of the threatened aircraft, a firstcommand indicator in each receiving aircraft energizable by occurrencesof the said first outputs threat to command the pilot at the saidaircraft to climb, and a second indicator in each aircraft energizableby occurrences of said second outputs thereat to command the pilot todescend, means for modifying subsequent transmissions from a commandedaircraft to reflect an altitude shifted in the direction of thecommanded maneuver and means in the said other threatened aircraftresponsive to receipt of the modified transmission to produce areciprocal maneuver command.
 5. In the cooperative collision avoidancesystem of claim 4 the first indicator in the higher flying aircraft willbe energized to command the pilot thereat to climb while the secondindicator in the lower flying aircraft will substantially simultaneouslybe energized to command the pilot thereat to descend in altitude.
 6. Inthe cooperative collision avoidance system of claim 4 wherein threeaircraft are flying on courses that are determined to representcollision threats to each other, the further improvement of means fordetermining in one of said aircraft that it is flying at the highestaltitude of the three aircraft, means for determining in a second one ofsaid aircraft that it is flying at the lowest altitude, said means fordetermining that said one aircraft is flying at the highest altitudeproducing an output to energize the first command indicator thereat, themeans for determining that said second aircraft is flying at the lowestaltitude producing an output to energize the second command indicatorthereat, the third aircraft including means for producing a secondoutput thereat with respect to the highest flying aircraft and a firstoutput thereat with respect to the lowest flying aircraft, said firstand second outputs produced at said third aircraft simultaneouslyenergizing the first and second command indicators thereat.
 7. In thecooperative collision avoidance system of claim 4 wherein the bi-stablemeans in each aircraft for determining which aircraft is flying higherand which is flying lower include a flip-flop circuit.
 8. In thecooperative collision avoidance system of claim 4 including meansresponsive to the energizing of the first command indicator only tomodify the transmitting means in the associated aircraft so that theaircraft will transmit altitude signal encoded to represent an altitudethat is higher by some predetermined altitude than the actual altitudeof the said aircraft.
 9. In the cooperative collision avoidance systemof claim 4 including means responsive to the energizing of the secondcommand indicator only to modify the transmitting means in theassociated aircraft so that the aircraft will transmit altitude signalsencoded to represent an altitude that is lower by some predeterminedaltitude than the actual altitude of the said aircraft.
 10. In thecooperative collision avoidance system of claim 6 wherein said means insaid one aircraft for energizing the first command indicator thereatinclude means to modify the transmitting means at said one aircraft sothat the transmitting means will transmit altitude signals encoded torepresent a higher than actual altitude condition therefrom, and whereinsaid means in said second aircraft for energizing the second commandindicator thereat include means to modify the transmitting means at saidsecond aircraft so that the transmitting means will transmit altitudesignals encoded to represent a lower than actual altitude conditiontherefrom, the simultaneous energizing of the first and second commandindicators in said third aircraft having no effect on the encodedaltitude signals being transmitted therefrom.